The present invention relates to storage of information, and in particular to high-density computer storage.
At present, the magnetic hard disk is the predominant device for mass data storage in microelectronics applications. The ever-growing demand for storage capacity has engendered dramatic increases in bit density and read/write speed, even as the price per stored megabyte has fallen. Whereas in 1990 the areal density of state-of-the-art hard disks was less than 0.1 Gbit/in2, currently available disks may have areal densities in excess of 5 Gbits/in2. It is expected that design scaling and the move toward giant magnetoresistive heads will push areal densities into the upper tens of Gbits/in2. This growth rate cannot be sustained indefinitely, however, and conventional scaling is expected to peak in 2006. Of course, this technological limitation will not reduce the demand for greater storage capacity in less space.
Numerous alternatives to magnetic storage have been proposed. Nanoimprintation, for example, has been used to fabricate 400 Gbit/in2 read-only (compact disc) devices and 45 Gbit/in2 read-write devices (see Krauss et al., Appl. Phys. Lett. 71:3174 (1997); Wu et al., J. Vac. Sci. Technol. B 16:3825 (1998); and Cui et al., J. Appl. Phys. 85:5534 (1999)). Read-write heads based on scanning probes have achieved areal densities of 400 Gbit/in2 (see Binnig et al., Appl. Phys. Lett. 74:1329 (1999); Mamin et al., Appl. Phys. Lett. 69:433 (1996); Chui et al., Appl. Phys. Lett. 69:2767 (1996)). Efforts have also been made to utilize scanning probe microscopes to store data by surface modification (see Betzig et al., Science 251:1486 (1991); Barret et al., J. Appl. Phys. Lett. 70:2725 (1991); and Terris et al., Appl. Phys. Lett. 68:141 (1996)). None of these techniques, however, has approached the current goal of a terabit per square inch.
While devices operating on the atomic or molecular scale surpass this threshold, they are generally not suited for commercial data storage due to stringent low-temperature requirements or the need to operate under vacuum conditions. For example, the cryogenic scanning tunneling microscope (STM) has been used to move single atoms (see Stroscio et al., Science 254:319 (1991)), and the vacuum STM to align C60 molecules on copper lattices (see Cuveres et al., Appl Phys. A 66:S669 (1998)).
The present invention utilizes an atomic force microscope (AFM) tipped with a single-wall conductive nanotube, and preferably operated in the xe2x80x9ctappingxe2x80x9d mode, to write bits onto a metal substrate by oxidizing the surface. The oxidized microregions project above an otherwise flat surface, and can therefore be detectedxe2x80x94that is, the written bits can be readxe2x80x94using the same AFM arrangement.
In a preferred embodiment, the AFM tip is provided with a single-walled carbon nanotube, and is operated to oxidize an atomically flat titanium surface. Using this arrangement, bit densities of 1.6 Tbits/in2 have been achieved. Moreover, the extreme hardness and cylindrical shape of the SWNT element avoids significant tip wear, thereby preventing bit degradation during the write process and minimizing tip convolution during read operations.